lavf/matroska: Adding the new SeekPreRoll element

In order to encapsuate Opus in Matroska, there is a new element
that has been added to the Matroska Spec, SeekPreRoll. It has the
duration in nanoseconds that has to be decoded before every seek.
Spec: http://matroska.org/technical/specs/index.html#SeekPreRoll
Proposal for encapsulateing Opus in Matroska:
http://wiki.xiph.org/MatroskaOpus

Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>